Firefox will soon be able to block third-party advertising cookies by default, preventing ad networks from tracking users' browser activity.

Advertisers use cookies to track users' Web activity to deliver more-targeted ads. The new patch will allow cookies from sites users actively visit but block those from third-party sites that haven't been visited by the user.

Firefox users have long had the ability to manually disable the cookies, but the patch will allow the browser to automatically perform the task. Contributed by Jonathan Mayer, a researcher at Stanford, the patch is expected to be released in Firefox 22 on April 5.

The move is not exactly revolutionary. While Google's Chrome and Microsoft's Internet Explorer allow third-party cookies, Apple's Safari does not.

"In short, the new Firefox policy is a slightly relaxed version of the Safari policy," Mayer said in a blog post Friday.

The announcement did not sit well with advertising executives. Mike Zaneis, the Interactive Advertising Bureau's senior vice president, tweeted that the new policy was a "nuclear first strike against the ad industry."

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After the U.S. Federal Trade Commission requested a mechanism to block online tracking, Mozilla offered Do Not Track technology to prevent Web pages from tracking people's online behavior for advertising purposes. However, efforts to make it a standard have stalled over disagreements about how to enable it.

In an initial approach, DNT would be active only if a person specifically enabled it. However, advertisers said they'd ignore the DNT setting if it was enabled by default.